This disclosure relates in general to photolithography and, but not by way of limitation, to resist free patterning (RFP) photolithography amongst other things.
Generally speaking, photolithography is a process used in microfabrication to selectively remove portions of a thin film. Typically, light is used to transfer a geometric pattern from a photomask or a reticle to a light-sensitive chemical photoresist on the substrate. A series of chemical treatments then engraves the exposure pattern into the material underneath the photoresist. In a complex integrated circuit (for example, modern CMOS), a wafer can go through the photolithographic cycle up to 50 times.
A traditional photolithography process may include the following steps: preparation, photoresist application, exposure, developing, etching, and removal. A wafer may be prepared by heating it to a temperature sufficient to drive off any moisture that may be present on the wafer surface. Wafers that have been in storage must be chemically cleaned to remove contamination. A liquid or gaseous adhesion promoter, such as, for example, hexamethyldisilazane (HMDS), may be applied to promote adhesion of the photoresist to the wafer.
The wafer may then be covered with photoresist by various deposition techniques such as, for example, spin coating, chemical vapor deposition, atomic layer deposition, physical vapor deposition and variants thereof. The photoresist-coated wafer may then be soft-baked or prebaked to drive off excess solvent. After prebaking, the photoresist may be exposed to a pattern of intense light. In response to the light exposure, a positive photoresist becomes less chemically robust, while a negative photoresist becomes more robust. This chemical change allows some of the photoresist to be removed by a developer solution. A post-exposure bake is often performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light.
Wafer may then be hard-baked. In some cases the hard-bake is performed at 120° to 180° C. for 20 to 30 minutes. The hard bake solidifies the remaining photoresist, to make a more durable protecting layer in future ion implantation, wet chemical etching, or plasma etching. Following the hard-bake, the wafer is etched using, for example, a liquid (“wet”) or plasma chemical agent that removes the uppermost layer of the substrate in the areas that are not protected by photoresist.
Following the etching, the photoresist can be removed from the substrate. A liquid resist stripper may be used to chemically alter the resist so that it no longer adheres to the substrate. Alternatively, photoresist may be removed by ashing, which oxidizes the photoresist with an oxygenated plasma. Various other techniques and/or modifications may be performed in a photolithography system.
These traditional photolithography processes are time and process intensive. Embodiments of the present invention address decrease the process and time demands of a photolithography system.